Electromagnetic translating device



pril M, 1936. J. A. MILLER 2,037,255

ELECTROMAGNETIC TRANSLATING DEVICE Filed March 21, 1931 2 Sheets-Sheet 1 INVENTOR JANA-"319. MILLER A TTORAVEY April 14, 1936. v A LE 2,037,255

ELECTROMAGNETIC TRANSLATING DEVICE v Filed March 21, 1931 2 Sheets-Sheet 2 Bela/I've hypo/me, 0 6 (Arb/ frar .rca/e) flay/1 11c [yr/e: per .recand [N VEN T OR JA/YIJ' A MILLER ATTORNEY Patented Apr. 14, 1936 i NlTED STATES PATENT OFFICE 2,037,255 I ELECTROMAGNETIC TRANSLATING DEVICE James Arthur Miller, Forest Hills, N. Y., assignor to United Research Corporation, a corporation of Delaware This invention relates to recording or reproducing sound and more particularly to an improved electromagnetic device for translating electric currents, representing sound, into phonographic records or the like, or for reproducing sound from such records.

Anobject of this invention is to make a phonographic record of sound in which the individual impulses are represented in accordance with their relative values in the original sound, throughout the important range of audio frequencies.

Another object is to make a record cutter which will haveany desired response'characteristic.

Another object of this invention is to reduce the power required to actuate an electromagnetic translating device for recording soundand to increase the sensitivity of the device for reproducing sound.

Another object is to reduce the audio frequency power formerly considered necessary for making a phonographic sound record of substantially uniform character throughout the useful audio frequency range.

Still another object of this invention is to coordinate predetermined natural vibration characteristics of the moving parts of an electromagnetic record cutter, suitably damped, with the characteristics of an attenuation network, to ob tain a desired response characteristic for the combined cutter and network.

A further object of this invention is to make a sound recorder, the record-making element of which will have substantially constant integrated velocity for the important audio frequencies when energized by equal voltages at said frequencies.

Recording and reproducing devices heretofore used have not been entirely satisfactory because their non-linear characteristics result in distortion of the sound to be recorded or reproduced. This distortion is due to resonant vibration of the armatures of said devices, at least to'a great extent. Attempts have been made to so design moving elements or armatures that their resonant frequencies will be outside of the important range of audio frequencies. Such attempts have not been successful because the important range of audio frequencies for high quality recording and reproduction of sound is very wide. This range is considered to include all frequencies from approximately 100 cycles to approximately 5000 cycles, and wider ranges are considered important if the recording or reproduction is to be of very high quality. It has been found that curves representing the response-frequency characteristics of ordinary electromagnetic translating devices have at least one prominent peak, and sometimes more. The construction of these devices has been such that an attempt to reduce one peak results in the displacement or increase of a peak in some other part of the curve, due, apparently, to the rigid construction of their moving elements, Attempts have been made to damp the excessive vibration of an armature at its resonant frequency'by pressing blocks of rub-- ber or other "soft material against the armature or by immersing the entire moving structure in oil. It has been found, however, to obtain a flat response curve by damping alone, that the sensitivity of the device is reduced to such an extent that an excessive amount of power is required to drive it when used as a recorder, oradditional amplification is required when the device is used as a reproducer or pick-up.

According to the present invention and in order to accomplish the objects above mentionedpa translating device is provided in which the movingelement is-not rigid but has a plurality of degrees 'offreedom of vibration, in the audio range, which may be independently controlled. It has been discovered that if parts "of an armature are proportioned to have desired resonant frequencies individually, and if proper coupling is provided between the armature parts, that the transfer of energy between them may be controlled. It has @1 0 been discovered that a desirable way to effect this energy transfer is to establish a harmonic relation between the resonant frequencies of various parts of the armature. A moving element constructed in accordance with this invention will have a plurality of degrees of freedom of vibration, thefrequencies of which are determined by the mass, resilience and physical dimensions of its parts. A curve showing the response frequency characteristic of an electromagnetic device having a moving element in accordance with this invention will show, if corrective means are not employed, a plurality of peaks corresponding to the natural periods of vibration of the armature as a whole and its parts. There may be other peaks arising from the interaction of one part of the moving element upon another part through the coupling between them. The higher peaks and especially those occurring in the lower frequency range may be reduced by proper damping, but the amount of damping required is much less than is needed to obtain a fairly fiat curve by damping alone. Damping is preferably accomplished by immersing the moving element in a liquid. The amount of damping may be readily controlled by associating adjustable surfaces with surfaces of the moving element. The remaining and usually broad peaks may then be reduced by placing an attenuation network in series with the actuating coil of the device.

A very desirable response-frequency curve for a sound recorder or reproducer is a substantially straight horizontalline over a large part of the important audio frequency range, which tapers off at the low frequency end. Such a characteristic may be obtained, with high sensitivity; by taking advantage of and properly selecting the resonant frequencies of the moving element as explained above. A sound: recorder-or reproducer constructed in accordance with this invention will have such a characteristic curve if the moving element is designedzto have. a plurality of resonant frequencies at selected points. in the audio frequency range, sufficient damping is provided to reduce-the amplitude of; the resonant Virbrations without, however, unduly affecting'the sensitivity of thedevice, and the remaining peaks are compensated by the selective transmission of the attenuation network.

Other objects and structural details of; this invention will be apparent from the-following description when read in; connection with the accompanying drawings, wherein:

Fig. 1 is a diagrammatic view of-a reed and is intended to illustrate aprinciple. involved inthis invention;

Fig. 2-is a perspective View of acomplete trans.- lating device made in accordance withthis invention;

Fig, 3 is a perspective view of the moving element of the device shown in Fig. 2;

Fig". 4 Ba crosssection, on.-the. line 4-4, of-the. moving element of Fig. 3;

Fig. .5 showsthe cover; of the device .shownr in Fig. 6 is arcrosssection-of-the. device shown in Fig. 2 with some parts,- removed for: clearnessandshows, diagrammatically, a. circuit. connected .to the-,actuating'coil of the device;

Fig. 7' is a: side elevation, on. a; larger scale;- ofthe adjustable dampingrmeansof the device shown in Figs. 2 and 6 Fig.- 8 illustrates adjusting and holdingzmeanstforthepole pieces of the deyiceashown in Fig. .6;

Fig. 9 shows curves ofresponse-versus frequency which have beenobtained from the de'-- vice illustrated-in Fig; 2.

If a reed, indicated generally-by reference character- |,.Eig. 1, has a plurality of. sections a, b and:

0 having different cross sections, eachof the sectionsea, b and.c may-have a different natural period: of vibration witlra node at the pointof juncture with the next section forlrand c orateach section, but the reed. will have resonant vibrations. due to the vibrations of two or more sections taken together and due to combinations of sectional vibrations as influenced by the coupling between the sections.- Such a reed will, therefore, have aplurality of resonant frequencies ofvibration; It has been discovered that if the natural vibrationperiods of two or more sec'- tionsare harmonically related, the-transfer of" energy between: them is a harmonic. transfer The res-- and that if energy is expended at the fundamental frequency, it is not present at the harmonic frequencies, or, stated in another way, if the energy is properly controlled at the low frequency, the amplitude of vibration may be controlled thereby at the harmonic frequencies.

These discoveries have been put to a useful purpose in the design of the moving element for a translating device or cutter such as shown in Fig. 2. It should be understood that an electromagnetic translating device which will cut a record may also be used as a reproducer. The moving element or armature of the device shown in Fig. 2 isillustrated in detail in Fig. 3. The armature proper is indicated by reference numeral l6 and has a support, preferably in the form of a torsional spring l I, provided with holes l2, through which screws may be passed to support the moving structure in the frame of the electromagnetic device. It will be noted that the armature IOis relatively thick at the point of juncture withspring Island at its end portions l3 and andlB. To the end portion l3, a relatively thin vanel4 isattaohedbutthe vane may be integral with. the armature ID, as shown in the cross sectional view (Fig. 4). To the. opposite end iii of the' armature is attached in any suitable manner a needle'orra stylus support I! to which a stylus I8Jmay be secured. The portion Ill with ends I3 and: I6 of'the. moving element is the only portion which must be made of magnetic material, preferably steel containing about 4% silicon. The other parts-ofithe moving element are preferably non-magnetic, such as hard brass or bronze for spring. II and aluminumr. for stylus support I! and vane M. The stylus I8 is usually cut from a sapphireand has an accurately ground point so that it; will .cutsa groove with smooth walls of the desired shape.

IrrFig. 2, the stylus support I! and the stylus I8Jare shown protruding from the bottom of the cutter. Adjacent the stylus is a pin or ball 49 ina; suitable holder 2| preferably in the form of a bent lever hinged on a rod 22. In thesoundrecording operation, the smoothly rounded pin I9 restsuponthe wax, or other blank on which thetsound is to be recorded, in advance of the cutting stylus l8and determines the depth of cut. The position of the pin I9 relative to the stylus I8is adjustable by a screw 23 acting on the opposite end of the lever 2 la'which is held against the screw 23 by spring means (not shown). Rod'22 and lever 2| are detachable so that pin l9 may be easily refinished or replaced. A thermometer 24 may be mounted on the outside of the case 25 with its bulb in close contact therewith to indicate the temperature of the device when in use. Extending'portion 26 provides means for securing the cutter to a' suitable supporting arm (not shown).

In Fig; 6, the moving'element is shown in section and in relation to other parts of the cutter. A magnet 28, preferably a small but strong permanent magnet, provides flux in the air gaps adjacent the portions I 3 and 16 of armature Ill. These air gaps are formed by magnetic pole pieces 30 and 3| having surfaces in close contact with the ends of magnet 28, and having other surfaces disposedjadjacent the thick part it of the armature. The air gap adjacent end l6 of the armature is adjustable by sliding the pole pieces 30 and 3| toward or away from the armature and securing them in position on the plate 32 (Fig. 8). The air gap adjacent end l3 of the armature may beadjustedby-slidably positioning plates 33 and and as illustrated in the drawings are shownv 34 on pole pieces 3|] and 3|, respectively, and securing them in position by meansof screws 35 and 36, or other suitable means. Disposed over the vane I4 is a member 38 having a slot therein. Member 38 is movable toward and away from the armature preferably by screw member 39 (Fig. 7) which passes through the cover 40 ofxthe cutter and has a head or knob 4| on the outside of the cover. Member 38 is provided with suitable guiding means, such as angle plates 43 and 44, and a pin 45 slidably mounted in angle plate 45. By varying the adjustment of member 38 by means of screw 39, varying portions of the vane l4 may be disposed within the slot in the member38. Angle pieces 43, 44 and 4B are secured'to a support which, in turn, is secured to the cover 40.

An actuating coil of a suitable number of turns and size wire is disposed between the pole pieces 30 and 3| and surrounds the armature Ill. Leads 5| and 52 serve to connect coil 50 to a suitable source of audio frequency power, or to a circuit carrying the current which represents the sound to be recorded. An attenuation network, indicated generally'at 53, and which may comprise resistance 54, condenser 55, and inductance 56, all connected in parallel, may be inserted in lead 52.

After the parts are assembled in the case 25 (Fig. 2) it is filled with oil, or other damping medium, through an opening in the cover 40 (Fig. 5). The cover 40 is provided with terminals 51 through which connections may be made to the coil 50. The oil in case 25is prevented from running out by a suitable gasket 58 surrounding the stylus support ll. Gasket 58 may be made of rubber or other elastic or flexible material which will permit movement of the stylus. The gasket is suitably secured to the casing 25, as by clamping ring 59.

Some of the principal dimensions of a'moving element which has beenfound to give satisfactory results, are as follows:

Width of armature l0 and ends I3 and "L. .160

Thickness of ends 53 and I6 Thickness of upper portion of armature IL. .045 Thickness of lower portion of armature l0 .042 Length of upper thin portion of armature H3. .155 Length of lower thin portion of armature I0- .198 Diameter of stylus support ll .052

Vane l4 .435 x .300 X .0045

(All the above dimensions are in inches) Representative response characteristics of a cutter made in accordance with this invention in Fig. 9. The curve indicated by I is a response characteristic of the cutter with no oil in the. case 25 and with the actuating coil 50' connected.

7 and vane I4, supported by the stiffness of spring H, which constitutes a resilient fulcrum. This resonant period may be increased by stiffening spring ll and by decreasing the mass of i the armature or the parts attached thereto. It may be decreased by decreasing the stiffness of spring It will be noted that this curve shows II, or increasing the, mass of the armaturev and. attached parts.-

. The .3600 cycle resonant peak is due to the vane l4, moving with reference to armature end portion l3. -Its period may be controlled by changing its mass and stiffness, a long thin vane having a lower period than a short thick vane. I The 7200 cycle peak is likewise due to the proportions of the longer end of thearmature extending between its center and end portion l6. The 9000-cycle peak is due to the shorter: end of the armature bar extending from its center to end portion I3.

The proportions as above given provide for an armature system which has a natural resonant peak of 900 cycles due to the bodily movement of the entire armature, stylus, and'damping vane, whereof said vane has a natural resonant peak which is the fourth harmonic thereof, with the longerend of the armature having a resonant peak which is the eighth harmonic thereof, and

a shorter end of the armature having a peak which is the tenth harmonic thereof.

Curve II is a characteristic of the same cutter.

under the sameconditions but with the case filled with castor oil. It will be noted that the resonant peaks have been smoothed out or merged, but a characteristic of this type is not entirely satisfactory because of the relatively high response in the middle range of frequencies. Curve III was obtained. with the same cutter and oil but with an attenuation network, as shown in Fig. 6, connected in series between the actuating coil and the energy source. This attenuation network comprised a resistance of 1000 ohms, a capacity of .03 microfarad, and an in-.

'ductance of 250 millihenries. The series resistance 54 tends to reduce the response equally throughout the entire audio frequency range but is more effective at the very low frequencies. The inductance 55 in parallel with resistance 54 increases the relative response at the low fre-.

quencies because of the relatively low impedance of the inductance at these frequencies. The response at the upper end of the scale isincreased due to the relatively low impedance of the condenser 55 at the high frequencies. The

small, broad peak at the upper end of curve III is probably due to the efiect of series resonance between condenser 55 and actuating coil 50. If no attenuation network were used and sufiicient damping were applied to the moving element to obtain a curve as -flat as curve .111,

the relative response would have been consider ably less than that obtained by the combination of moderate oil damping and. the attenuation network, as shown in curve III.

The initial response of the device is made very high due to the plurality of resonances, as shown by curve I, so that sufl'icient damping can be applied "to merge the resonant peaks, but with the sensitivity of the device maintained at a high level. The net result is a cutter whioh has high sensitivitywith a practically fiat characteristic curve. teristic curve at the low frequency end is desirable to prevent excessive swings of the recording sty- 2 lus at the low frequencies, which would result in cross-overs in the record grooves. This falling offiscompensated in the reproducing operation because pick-up devices ordinarily used are more sensitive at the low frequencies.

Characteristic curves in whicha desired band.

or bands of frequencies are relatively high or low may be obtained from translating devices made in accordance with this invention by arranging the resonance peaks of the'various parts ofthe moving elements to fall either within or without the desired bands depending upon whether the said bands are to be increased or reduced, respectively. Sufficient damping may then be provided to reduce the resonant vibrations to the desired level and compensating networks may be connected in circuit with the actuating coil to reduce any undesired frequency bands.

The usual method of using the device shown in Fig. 2 is as follows: The device isconnected to the output of a suitable amplifier, the input of which is connected to a microphone. The cutter is supported by a suitable. arm which grips the extending portion 26' and which is arranged to guide the cutter over a wax record blank. The sapphire pin I9 rides on the smooth surface of the'moving wax 60, and the stylus l8, which is slightly lower than the pin l9, cuts a groove in the-wax. The variations of current in coil 50 produce corresponding vibrations of the stylus I8 which, in turn, makes an undulating groove in the wax. If the cutter is then connected to the input of a suitable amplifier and sound reproducer and its supporting arm allowed to move freely, the. stylus will follow the groove in the wax and the sound recorded thereon will bereproduced.

The degree of damping of the oil depends on its temperature. After the temperature characteristic of the cutter is determined, uniform damping may be obtained by adjusting thescrew 39 in accordance with the temperature. of the oil, as indicated by thermometer 24.

While thereis here described but asingle embodiment of'the device of this invention, it is adapted to still other modifications therefrom without departure from the inventive concept disclosed, and it is therefore desired that only such limitations shall be imposed upon the. appended claims as are stated therein or required by the prior art.

I- claim:

1. A translating device comprising a pair" of magnetic poles providing two air gaps therebetween, an armature having a portion thereof disposed in each of said gaps, and means for resiliently supporting said armature between said poles; said armature having portions thereof extending beyond said gaps, the armature and its extending portions having several sections thereof differently proportioned with respect to mass, dimensions and resilience, whereby said armature has a plurality of harmonically related. natural periods of vibration.

2. A translating device having an armature, portions of which have different masses and cross sections of such value that the natural frequency of vibration of one portion of said armature is harmonically related to the natural frequency of vibration of another portion thereof.

The gradual falling off of-the charac-- 3. The. combination of an armature wherein thesrelation of mass tostiifness isdifferent: fordiiferent portions thereof, said portions having their natural frequencies of vibration in harmonic relation in the audio frequency range, means for partially damping the vibration of said portions, and means for producing a field of force for said armature.

4. Anelectromagnetic translating device comprising a. pair of'adjacent magnetic poles forming two air gaps therebetween, an armature disposed in said gaps, a support for said armature comprising a torsion spring, the lengths of those portions of said armature between the point of support'and'said pole pieces being unequaland each of said armature'portions being relatively thicker opposite the said poles than between the poles andthe point of support, a relatively thin vane protruding from one end of said armature, and 'aistylus support protrudingfrom the opposite end of said armature.

5. A translating device as defined in claim 4 inwhich said armature portions and said vane are formed from .onepiece of magnetic material.

6. A translating device as defined in claim 4 in which thenatural vibration periods of said vane and said stylus support are harmonically related and the natural period of vibration of atleast one of said portions of the'armature is harmonically. related to the said natural periods of the stylus support and vane.

7. Anarmature for an electromagnetic device comprising a body portion, a relatively thin vane protruding from oneend of said body portion, a stylus support protruding from the opposite end of said body portion, and astylus secured tosaid support, the natural vibration periods of said vane and said stylus support with its stylus being.

harmonically related.

, 8. An acoustic. device comprising an electromagnet having an armature, .a spring support for said armature, a portion of said armature being vibratile with respect to the armature as a whole, the stiffness of said spring support alsov the stiffness of said vibratile'portion and the masses of said armature as a whole and of said portion thereof being so related that the natural period of vibration of said armature as a whole is harmonically related to the natural period of .vibration of said portion thereof, whereby the harmonic vibrations are overemphasized.

9. An acoustic device according to claim 8, wherein said spring support is arranged at one side of the center of said armature whereby, the ends of said armature beyond. said. spring support are of unequal length.

10. An acoustic device according to claim 8' wherein saidspring support is arranged at one side of the center of said armature whereby the ends of said armature beyond said spring support are of unequal length, and wherein the short end of said armature is provided with a damping Vane and the longer. end of said armature is provided with a stylus.

11. An electromagnetic translating device comprising an armature having a plurality of vibratile sections having different mass-stiffness ratios respectively with harmonically related natural periods of vibration in the audio frequency range;

means for partially damping the vibrations of said sections, and means for producing a magnetic field for said armature.

12. A record cutter comprising an'armature having a plurality of vibratile sections of different crosssectional areas respectively wherein the 13. A translating device having an armature, portions of which have different masses and cross sections of such values that the natural frequency of vibration of one portion of said armature is harmonically related to the natural frequency of 5 vibration of a plurality of other portions thereof.

JAMES ARTHUR MILLER. 

